The present invention relates to a rare-earth element oxysulfide phosphor suitable for use in a radiation detector for detecting X-rays, xcex3 rays and the like and particularly for use in the radiation detector of an X-ray CT apparatus, a positron camera or the like. The present invention also relates to a radiation detector and an X-ray CT apparatus using the phosphor.
As the radiation detectors used in X-ray CT apparatuses and the like there have conventionally been used ones combining a xenon gas chamber or BGO (bismuth germanium oxide) single crystal and a photomultiplier tube or combining CsI:T1 single crystal or CdWO4 single crystal and a photodiode. In recent years, however, rare-earth-system phosphors with high radiation-to-light conversion efficiencies have been developed as scintillators and radiation detectors combining such a phosphor with a photodiode have been put into practical use.
Rare-earth phosphors include rare-earth element oxide phosphors having a matrix mainly of L2O3 (L representing an element such as Y or Gd) and containing a small amount of activator (Japanese Unexamined Patent Publication No. 64 (1989)-38491, for example) and rare-earth element oxysulfides having a matrix of L2O2S and containing a small amount of activator. Although the former phosphors can be produced as ones of cubic crystal system and therefore have the advantage of excellent transparency, they have the drawback of being inferior to the latter phosphors in luminous efficiency.
In contrast, rare-earth element oxysulfide phosphors are high in luminous efficiency. Japanese Patent Publication No. 60(1985)-4856, for example, teaches Gd2O2S:Pr, Ce, F and Japanese Patent PublicationNo. 59(1984)-38280 teaches (Y, Gd, La or Lu)2O2S:Tb, Ce. The emission peaks of these phosphors differ depending on the activator. A phosphor using Pr as activator emits green light and a phosphor using Tb as activator emits blue or green light. Phosphors using Eu as activator emit red light and are used as color TV phosphors (Japanese Patent Publication No. 47(1972)-13243).
Properties generally required of a scintillator material used in a radiation detector include short afterglow, high emission efficiency, high X-ray stopping power and chemical stability. Large afterglow is particularly a problem in X-ray CT applications, for example, because it makes information-carrying signals indistinct in the time-axis direction. Very small afterglow is therefore required.
Phosphor afterglow generally includes primary afterglow and secondary afterglow (long-afterglow component). The primary afterglow has a relatively short attenuation period (less than around 2 ms) but the secondary afterglow has a longer attenuation period that is particularly undesirable when the phosphor is used as a scintillator. When the secondary afterglow is large, information-carrying signals become indistinct in the time-axis direction. Secondary afterglow is thought to be caused by the contribution to emission of electrons and holes thermally released from traps formed by phosphor lattice defects. It can be reduced by reducing the number of defects becoming shallow traps or by adding another additive that essentially reduces the action of the shallow traps.
For example, in the case of the rare-earth element oxysulfide phosphor taught by Japanese Patent Publication No. 60(1985)-4856, whose luminous component is Pr, a phosphor capable of utilization as an X-ray CT scintillator is obtained by addition of Ce.
For medical diagnosis applications, however, a detector of still higher detector efficiency is desired in order to minimize the radiation dosage received by the human body while still securing excellent detector efficiency and high SN ratio. In addition, phosphors that use Pr or Tb activators have a problem of low overall detection efficiency of the radiation detector, despite high emission efficiency and short afterglow, since they emit blue or green light and therefore have poor wavelength matching with PIN photodiodes currently used as photodetectors in radiation detectors employed in X-ray CT and the like, owing to the fact that the PIN photodiode""s peak response wavelength is in the red region.
An object of the present invention is therefore to overcome these problems of the prior art and to provide a phosphor with very short afterglow and high emission efficiency that is particularly useful as a scintillator in X-ray CT and the like. Another object of the present invention is to provide a radiation detector that exhibits excellent wavelength matching between the phosphor and the photodetector and is high in detection efficiency (luminous output). Another object of the present invention is to provide an X-ray CT apparatus that is equipped with a radiation detector with very small afterglow and high emission efficiency as a radiation detector and can provide high-resolution, high-quality tomographic images.
In order to achieve the foregoing objects, the inventors conducted an intense study regarding rare-earth element oxysulfide phosphors having Eu as the luminous component and, discovering as a result that a phosphor of high emission efficiency and greatly reduced secondary afterglow is obtained by adding prescribed components, they arrived at present invention.
Specifically, the phosphor of the present invention is a phosphor represented by the general formula
xe2x80x83(L1-x-y-z-dEuxMyCezMxe2x80x2d)2O2S
where L is at least one element selected from the group consisting of Gd, La and Y, M is at least one element selected from the group consisting of Tb and Pr, and Mxe2x80x2 is at least one element selected from the group consisting of Ca, Sr and Zn. In addition, x, y, z and d are values falling in the ranges of 0.001xe2x89xa6xc3x97xe2x89xa60.06, 0 less than yxe2x89xa612xc3x9710xe2x88x925, 0 less than zxe2x89xa612xc3x9710xe2x88x925, and 0xe2x89xa6dxe2x89xa62.5xc3x9710xe2x88x924.
This phosphor is a rare-earth element oxysulfide phosphor having a matrix of L2O2S and containing Eu activator component. It absorbs radiation such as X-rays, gamma rays and nuclear radiation, exhibits Eu emission having peaks straddling 600nm, and exhibits numerous line emissions in the range of 450-700nm. When such a phosphor is used as the scintillator of a radiation detector, matching with the photodiode is extremely good and a luminous output can be obtained that is twice or more than that of the CdWO4 currently widely used as a scintillator for X-ray CT.
Any of Gd, La and Y can be used as the element L. Although two or more of these elements can be used, the X-ray stopping power can be maximized by replacing the L position totally with Gd. The emission characteristics remain substantially the same, however, even if part of the Gd is replaced with La or Y.
Eu is an element that serves as an activator (luminous component) in the phosphor of the present invention. The Eu content for generating Eu emission (x:number of moles replacing 1 mole of element L) is preferably 0.001 or greater. The Eu content x is defined as 0.06 or less for applications requiring high luminous output because a luminous output twice that of CdWO4 cannot be obtained when the Eu content x exceeds 0.06. More preferably, the Eu content x is defined as 0.002-0.03. About 2.5 times the luminous output of CdWO4 can be obtained in this range.
Element M and Ce lower the afterglow of the phosphor of the present invention. As pointed out earlier, it is thought that shallow traps produced by phosphor lattice defects contribute to secondary afterglow and that afterglow can reduced by adding another additive that essentially reduces the action of the shallow traps. The inventors found through their research that element M and Ce are elements capable of effectively reducing secondary afterglow in a rare-earth element oxysulfide using Eu as activator.
Either Tb or Pr can be used as the element M and part of Tb can be replaced by Pr. As no effect of reducing afterglow can be obtained with only one of element M and Ce, however, at least one member of element M and Ce are included.
Afterglow reduction effect can be obtained even with very small contents of both element M and Ce. Although increasing M content y and Ce content z proportionally reduces afterglow, it also tends to lower scintillator luminous output. Preferably, therefore, the content of each should not exceed 12xc3x9710xe2x88x925 in applications requiring high luminous output. When these elements are included within this range, a luminous output can be obtained that is twice or more than that of the CdWO4 and, moreover, afterglow can be reduced to a small fraction of that in the case where only Eu is contained or only Eu and Ce or Eu and M are contained.
While Mxe2x80x2 is not an indispensable element in the phosphor of the present invention, Eu emission can be increased by addition of element Mxe2x80x2. One or more of Ca, Sr and Zn can be used as the element Mxe2x80x2 for increasing Eu emission. Among these elements, Ca is particularly preferable for its effect of enhancing Eu emission by up to a maximum of 7%. The foregoing effect of the element Mxe2x80x2 can be obtained at a content up to 2.5xc3x9710xe2x88x924. Particularly at a content in the range of 0.3xc3x9710xe2x88x924-2.0xc3x9710xe2x88x924, emission can be improved by around 3%.
The phosphor of the present invention is not particularly limited with regard to crystal morphology. The process for producing other phosphors as single crystal reported in J. Appl. Phys., vol. 42, p3049 (1971) can be applied as the process for preparing the phosphor of the present invention as single crystal.
Since the phosphor of the present invention is a rare-earth element oxysulfide, a process that prevents diffusion of sulfur during production must be adopted as the production process. The hot isostatic pressing process (HIP process) is preferable. The HIP process adds a sintering agent to the starting material powder, packs and vacuum-seals the result in a metal container of pure iron or the like, and effects hot isostatic pressing. Li2GeF6 or the like can be used as the sintering agent. The hot isostatic pressing is conducted for around 30 min to several hours under conditions of a temperature of 900-1,900xc2x0 C., preferably 1,100-1,400xc2x0 C., and a pressure of about 900-1,800 atm. This enables the phosphor to be obtained as a dense sintered body of high optical transmittance.
The phosphor before HIP processing can be prepared as follows: mixing Gd2O3, Eu2O3, Na2CO3 and S, for example, to obtain a prescribed composition, adding small amounts of additive elements (Tb (Pr), Ce, Ca etc.) as salts such as nitrates, adding an appropriate flux component, e.g., K3PO4.3H2O, Li2B4O7 or the like, packing the result in an alumina crucible, covering the crucible, and conducting baking at about 1,350xc2x0 C. for several hours (around 3xc3x9710 hr). The scintillator powder baked in this manner is subjected to the foregoing HIP processing.
As oxygen and sulfur defects are present in the phosphor after HIP, once the phosphor has been cut to the desired shape it is preferably annealed in Ar gas containing a small amount of oxygen at around 1,000xc2x0 C.-1,300xc2x0 C. for about 15 min-120 min.
The phosphor produced in this manner is dense, high in optical transmittance, and low in loss of light by scattering. A radiation detector of large luminous output can therefore be obtained.
Although the phosphor of the present invention can be used in intensifying screens, fluorescent screens, scintillators and other general phosphor applications, it is particularly suitable for use in X-ray CT detectors, which require high luminous output and small afterglow.
The radiation detector of the present invention is equipped with a ceramic scintillator and a photodetector for detecting scintillator emission. The phosphor described in the foregoing is used as the ceramic scintillator. A photodiode such as a PIN photodiode or an avalanche photodiode is preferably used as the photodetector. These photodiodes have high sensitivity and short response. Moreover, as they have wavelength sensitivity from the visible light to near infrared region, they are suitable for their good wavelength matching with the phosphor of the present invention.
The X-ray CT apparatus of the present invention is equipped with an X-ray source, an X-ray detector disposed facing the X-ray source, a revolving unit for holding the X-ray source and the X-ray detector and revolving them about the object to be examined, and image reconstruction means for reconstructing a tomographic image of the object based on the intensity of the X-rays detected by the X-ray detector, which CT apparatus uses as the X-ray detector a radiation detector combining the aforesaid phosphor and a photodiode.
High-quality, high-resolution images can be obtained by utilizing this X-ray detector because the high X-ray detection rate makes it possible to achieve an approximate doubling of sensitivity compared with an X-ray CT apparatus using a conventional scintillator (such as CdWO4) and also because its afterglow is extremely small. As the size of the elements can be reduced, moreover, images with high spatial resolution can be obtained. In addition, the X-ray dosage received by the object can be made lower than when using a conventional X-ray CT apparatus.